Digital demodulator

Abstract

A digital demodulator capable of a high speed operation. An automatic frequency controller (AFC) estimates a frequency deviation Δω on the basis of a detected sample signal which is outputted at an oversampling interval by a receiving filter. The AFC can estimate the frequency deviation before a stable operation of a bit timing recovery circuit (BTR). The AFC generates a frequency deviation correcting signal for each decision timing on the basis of the estimated frequency deviation, and decision timing information from the BTR. A multiplier eliminates a frequency deviation component from an input signal in response to the frequency deviation correcting signal.

What is claimed is: 
     
       1. A digital demodulator for coherently detecting received digitally-modulated signals, comprising: (a) a quasi-coherent detector for quasi-coherently detecting each received signal on the basis of a reference signal and outputting a detected sample signal at each sampling timing;   (b) a decision timing generator for generating decision timing for said each received signal on the basis of the detected sample signal;   (c) an interpolator for interpolating the detected sample signal on the basis of information concerning the decision timing, and generating a decision timing signal;   (d) a frequency deviation estimator for estimating a frequency deviation between a carrier of said each received signal and the reference signal based on the detected sample signal received from the quasi-coherent detector, and generating a frequency deviation correcting signal for each decision timing received from the decision timing generator on the basis of the estimated frequency deviation;   (e) a frequency deviation corrector for correcting the decision timing signal, on the basis of the frequency deviation correcting signal;   (f) a phase estimator for estimating an initial phase component of an output signal of the frequency deviation corrector and generating a phase correcting signal on the basis of the initial phase component; and   (g) a phase corrector for correcting the output signal of the frequency deviation corrector using the phase correcting signal.   
     
     
       2. The digital demodulator according to claim 1, wherein the quasi-coherent detector includes: an analog-to-digital converter for converting said each received signal into a digital signal;   a digital quadrature detector for quasi-coherently and quadraturally detecting an output signal of the analog-to-digital converter on the basis of two reference signals whose phases differ from each other by π/2; and   a receiving filter for filtering an output signal of the digital quadrature detector.   
     
     
       3. The digital demodulator according to claim 2, wherein the receiving filter is a Nyquist filter. 
     
     
       4. The digital demodulator according to claim 1, wherein the reference signal has a frequency substantially equal to a carrier frequency of said each received signal. 
     
     
       5. A digital demodulator for coherently detecting received digitally-modulated signals, comprising: (a) a quasi-coherent detector for quasi-coherently detecting each received signal on the basis of a reference signal and outputting a detected sample signal at each sampling timing;   (b) a decision timing generator for generating decision timing for said each received signal on the basis of the detected sample signal;   (c) an interpolator for interpolating the detected sample signal on the basis of information concerning the decision timing, and generating a decision timing signal;   (d) a frequency deviation estimator for estimating a frequency deviation between a carrier of said each received signal and the reference signal based on the detected sample signal received from the quasi-coherent detector, and generating a first frequency deviation correcting signal for each decision timing received from the decision timing generator and a second frequency deviation correcting signal for each sampling timing, on the basis of the estimated frequency deviation;   (e) a first frequency deviation corrector for correcting the decision timing signal on the basis of the first frequency deviation correcting signal;   (f) a second frequency deviation corrector for correcting the detected sample signal on the basis of the second frequency deviation correcting signal;   (g) a phase estimator for estimating an initial phase component of the detected sample signal on the basis of an output signal of the second frequency deviation corrector, and generating a first phase correcting signal for the sampling timing on the basis of the initial phase component;   (h) a correcting signal modifier for modifying the first phase correcting signal on the basis of the decision timing and the first frequency deviation correcting signal, and generating a second phase correcting signal for the decision timing; and   (i) a phase corrector for correcting the output signal of the first frequency deviation corrector on the basis of the second phase correcting signal.   
     
     
       6. The digital demodulator according to claim 5, wherein the quasi-coherent detector includes: an analog-to-digital converter for converting said each received signal into a digital signal;   a digital quadrature detector for quasi-coherently and quadraturally detecting an output signal of the analog-to-digital converter using two reference signals whose phases differ from each other by π/2; and   a receiving filter for filtering an output signal of the digital quadtrature detector.   
     
     
       7. The digital demodulator according to claim 6, wherein the receiving filter is a Nyquist filter. 
     
     
       8. The digital demodulator according to claim 5, wherein the reference signal has a frequency substantially equal to a carrier frequency of said each received signal. 
     
     
       9. A digital demodulator for coherently detecting received digitally-modulated signals, comprising: (a) a quasi-coherent detector for quasi-coherently detecting each received signal on the basis of a reference signal and outputting a detected sample signal at each sampling timing;   (b) a decision timing generator for generating decision timing for said each received signal on the basis of the detected sample signal;   (c) an interpolator for interpolating the detected sample signal on the basis of information concerning the decision timing, and generating a decision timing signal;   (d) a differential detector for differential-detecting the decision timing signal;   (e) a frequency deviation estimator for estimating a frequency deviation between a carrier of said each received signal and the reference signal on the basis of the detected sample signal, and generating a phase rotation correcting signal indicative of an amount of phase rotation per delay time of the differential detector, on the basis of the estimated frequency deviation; and   (f) a phase corrector for correcting an output signal of the differential detector on the basis of the phase rotation correcting signal.   
     
     
       10. The digital demodulator according to claim 9, wherein the delay detector includes: a phase calculator for calculating a phase component of the decision timing signal, and outputting a phase signal;   a delay circuit for delaying the phase signal by the delay time; and   a subtractor for subtracting the phase signal delayed by the delay circuit from the phase signal of the phase calculator.   
     
     
       11. The digital demodulator according to claim 10, wherein the phase calculator comprises a coordinate transformer for transforming the decision timing signal into the form of polar coordinates. 
     
     
       12. The digital demodulator according to claim 9, wherein the delay detector complex-multiplies the decision timing signal of the interpolator by the decision timing signal delayed by the delay time. 
     
     
       13. The digital demodulator according to claim 9, wherein the frequency deviation, estimator generates a frequency deviation correcting signal for said each sampling timing on the basis of the estimated frequency deviation, the quasi-coherent detector includes a frequency deviation corrector for correcting the detected sample signal on the basis of the frequency deviation correcting signal, and the digital demodulator further comprises a selector for selecting either the phase rotation correcting signal or the frequency deviation correcting signal of the frequency deviation estimator.   
     
     
       14. The digital demodulator according to claim 13, wherein the selector selects the phase rotation correcting signal when the digital demodulator operates at a high speed, and selects the frequency deviation correcting signal when the digital demodulator operates at an ordinary speed. 
     
     
       15. The digital demodulator according to claim 9, wherein the quasi-coherent detector includes: an analog-to-digital converter for converting said each received signal into a digital signal;   a digital quadrature detector for quasi-coherently and quadratually detecting an output signal of the analog-to-digital converter on the basis of two reference signals whose phases differ from each other by π/2; and   a receiving filter for filtering an output signal of the digital quadrature detector.   
     
     
       16. The digital demodulator according to claim 15, wherein the receiving filter is a Nyquist filter. 
     
     
       17. The digital demodulator according to claim 9, wherein the reference signal has a frequency substantially equal to a carrier frequency of said each received signal.